Muscle synergy in several locomotor modes in chimpanzees and Japanese macaques, and its implications for the evolutionary origin of bipedalism through shared muscle synergies.

Recent evidence indicates that human ancestors utilized a combination of quadrupedal walking, climbing, and bipedal walking. Therefore, the origin of bipedalism may be linked to underlying mechanisms supporting diverse locomotor modes. This study aimed to elucidate foundations of varied locomotor modes from the perspective of motor control by identifying muscle synergies and demonstrating similarities in synergy compositions across different locomotor modes in chimpanzees and Japanese macaques.

Diagonal-couplet gaits on discontinuous supports in Japanese macaques and implications for the adaptive significance of the diagonal-sequence, diagonal-couplet gait of primates.

Objectives: Diagonal-sequence, diagonal-couplet (DSDC) gaits have been proposed as an adaptation to travel on discontinuously arranged arboreal branches. Only a few studies have examined primate gait adjustment to support discontinuity. We analyzed the gaits of Japanese macaques walking on the "ground" and two discontinuous conditions, "circle" and "point," to better understand the advantages of DSDC gaits on discontinuous supports.

Hip medial rotator action of gluteus medius in Japanese macaque (Macaca fuscata) and implications to adaptive significance for quadrupedal walking in primates.

The gluteus medius (GM) muscle in quadrupedal primates has long been thought to mainly act as a hip extensor. However, previous reports argue that it may be a prime hip medial rotator and functions to rotate the pelvis in the horizontal plane, suggesting the functional differentiation between the GM and other hip extensors as hamstrings. In this study, we aim to quantify the muscle actions of the GM and hamstrings using muscle moment arm lengths and discuss the functional differentiation among hip extensors.